The present invention relates generally to gas turbine engines, and, more specifically, to turbine blades therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel for generating combustion gases in a combustor. Various turbine stages extract energy from the combustion gases to power the engine and produce work.
A high pressure turbine (HPT) immediately follows the combustor and extracts energy from the hottest combustion gases to power the upstream compressor through one drive shaft. A low pressure turbine (LPT) follows the HPT and extracts additional energy from the combustion gases for powering another drive shaft. The LPT powers an upstream fan in a turbofan aircraft engine application, or powers an external shaft for marine and industrial applications.
Engine efficiency and specific fuel consumption (SFC) are paramount design objectives in modern gas turbine engines. The various turbine rotor blades and their corresponding nozzle vanes have precisely configured aerodynamic surfaces for controlling the velocity and pressure distributions thereover for maximizing aerodynamic efficiency.
The corresponding airfoils of the blades and vanes have generally concave pressure sides and generally convex suction sides extending axially in chord between opposite leading and trailing edges. The airfoil has a crescent profile in radial section, increasing rapidly in width from the leading edge to a maximum width region, and then decreasing in width gradually to the trailing edge.
The circumferentially or transversely opposite sides of the airfoils also extend radially in span from root to tip. The airfoils typically have thin sidewalls formed by casting of superalloy metals, with internal cooling circuits having various embodiments all specifically tailored for efficiently cooling the airfoils during operation while maximizing efficiency.
However, aerodynamic design of turbine airfoils is remarkably complex in view of the three dimensional (3D) configurations of the individual airfoils in complete rows thereof, and the correspondingly complex flow streams of the combustion gases channeled between the airfoils during operation. Adding to this complexity of design and environment are the special flow fields around the radially outer tips of the turbine blades which rotate at high speed inside a surrounding stationary shroud during operation.
The operating clearance or gap between the blade tips and the turbine shrouds should be as small as practical for minimizing leakage of the combustion gas flow therethrough while also permitting thermal expansion and contraction of the blades and shrouds without undesirable rubbing between the rotating tips and stationary shroud.
During operation, the blades in a turbine row drive the supporting rotor disk in rotation with the airfoil suction side leading the opposite airfoil pressure side. The airfoils typically twist from root to tip in the radial direction from the perimeter of the rotor disk, and the leading edges face upstream obliquely with the engine axial centerline axis to match the oblique discharge swirl angle of the cooperating nozzle vanes.
The combustion gases flow generally in the axial downstream direction, with a circumferential or tangential component first engaging the airfoil leading edges in one flow direction, and then leaving the airfoils over the trailing edges thereof in a different flow direction.
The pressure and suction sides of the airfoils have correspondingly different 3D profiles for maximizing differential pressure therebetween and energy extraction from the hot combustion gases. The concave pressure side and the convex suction side effect different velocity and pressure distributions thereover which correspondingly vary between the leading and trailing edges, and from root to tip. However, the combustion gases which leak over the airfoil tips in the required tip clearance perform little, if any, useful work.
Further complicating turbine blade design is the exposed blade tips which are therefore bathed in the combustion gases which leak thereover during operation, and require suitable cooling thereof for ensuring a long useful life of the turbine blades during operation.
Modern turbine blade design typically incorporates squealer tip ribs which are small radial extensions of the pressure and suction sides of the airfoil from leading to trailing edge. The tip ribs are typically rectangular in cross section and spaced transversely or circumferentially apart to define an open tip cavity atop the airfoil which has an integral tip floor that encloses the typically hollow airfoil and the internal cooling circuit therein.
The small tip ribs provide sacrificial material in the event of a tip rub to protect the tip floor and internal cooling circuit from undesirable damage. The tip ribs increase the complexity of the combustion gas flow field introducing local secondary fields which affect turbine efficiency, flow leakage, and tip cooling.
The primary flow direction of the combustion gases is in the axially downstream direction in the flow passages defined between adjacent blades. The axial flow stream also varies along the radial direction from root to tip of each airfoil. And, these axial and radial flow variations are further compounded over the airfoil tip where the combustion gases leak between the pressure and suction sides of each airfoil.
Accordingly, the prior art is replete with various configurations of turbine blade tips addressing different problems and performance considerations including turbine efficiency, tip leakage, and tip cooling. These three important considerations are interdependent at least in part, but the complex 3D flow fields over the different pressure and suction sides at the airfoil tip and between the leading and trailing edges renders quite complex the evaluation thereof.
Furthermore, it is common practice to continually improve existing blade designs, but any change in blade design, and particularly in the tip, is interdependent with other blade features and might adversely affect the blade. Typical blade design requires careful balance of competing objectives and the careful evaluation of particular operating conditions, and this further complicates modern blade design.
Features which provide benefit in one blade design do not necessarily provide benefit in other designs for which they were not originally designed. Predictability of performance of individual features is illusory when introduced in different blades designed for different operating conditions.
Even the typical operating conditions of a given gas turbine engine vary over output power from takeoff to cruise, and performance of the turbine blades also varies thereover, as well as over the typical change in tip clearance.
However, modern computational fluid dynamics (CFD) includes powerful software that improves the ability to mathematically analyze complex 3D flow streams in gas turbine engines and provides a mechanism from which further improvements in turbine blade design may be realized.
Various blade permutations may now be more readily analyzed to predict performance thereof, and whether design changes are beneficial or not.
Accordingly, it is desired to improve turbine blade tip design by reducing tip flow leakage, or increasing turbine efficiency, or increasing energy extraction, or improving tip cooling, or any combination of these factors either separately or together.